The present invention relates to a turbo-molecular pump.
Turbo-molecular pumps are known wherein an exhaust action is effected by virtue of the combination of stationary blades and rotor blades adapted to rotate at high speed, the both the blades being adapted to cooperate with each other to impart momentum to gaseous molecules rushing in against the blades under a low-pressurized condition wherein the incidence of collisions between gaseous molecules is negligible.
The adoption of magnetic bearings in such a turbo-molecular pump enables non-contact support of the rotating shaft of the pump, and thus the life of the bearing can be beneficially made semipermanent.
Referring to FIG. 5, a prior art turbo-molecular pump having magnetic bearings will now be described. A fixed shaft 2 is provided in a casing 1, and a rotor 3 is placed over the shaft 2. Stator blades 4 are provided on the inner wall of the casing 1.
The fixed shaft 2 is provided thereon with active radial magnetic bearings 5, 5 and a motor 6, by means of which the rotor 3 can rotate at high speed in a non-contact manner. In FIG. 5, reference numerals 7, 7 respectively, denote touch-down bearings adapted to mechanically support the rotor 3 in case of failure of the magnetic bearings 5 and a rotor cover fixed to the rotor 3.
In a turbo-molecular pump of the type described above, vibrations caused by the residual unbalance that subsists during rotation, torque generated by the motor 6, shocks exerted on the casing 1 and so on disturb the control system of the magnetic bearings 5 as disturbance. And thus these disturbances act to vibrate the fixed shaft 2 at the natural frequency of the fixed shaft 2 so vibrated sometimes generates bending vibrations in the fixed shaft itself, which is shaped like a cantilever fashion, as shown in FIG. 6. The relationship between the amplitude and forced frequency ratio of these bending vibrations is shown in FIG. 7, and a simplified model of this structural system can, as shown in FIG. 8, be defined as having one degree of freedom.
These vibrations can constitute an adverse factor that makes the control system unstable, and self-excited or forced vibrations are sometimes generated in the pump main body. This serves to generate excessive vibrations in the pump main body and then to make it impossible for the magnetic bearings 5, 5 to support the rotor 3 in a stable manner, the functional reliability of the turbo-molecular pump thus being lowered.
The present invention has been contrived to solve the above stated drawbacks inherent to prior art pumps, and an object thereof is to provide a turbo-molecular pump that generates a reduced level of vibration during operation and that is reliable in functional terms.